The present invention relates to a range finder and, particularly, to a range finder of so-called active type based on triangulation for distance measurement and adapted for use with a compact equipment such as a still camera.
Most of modern cameras with lens shutter are equiped with an automatic focusing device of so-called active type which measures the distance to the object by emitting an infrared light beam to the object and detecting a reflected light beam. Among such devices there is proposed a range finder in which a light beam reflected from the object is focused by a reception lens, the focal plane is scanned by a moving split photosensor to detect the image of the reflection light beam, and the distance to the object is determined from the position of the image. This system operates to detect the image position in response to equal outputs of two splitted sensors, allowing a constant measuring accuracy for a wide range of reflectivity of objects. Moreover, in case the device is applied to a camera, a signal indicating whether the object image is front or back of the focal plane can readily be produced by using the two sensors, that cannot be achieved by other system using a single sensor for range finding.
On the other hand, however, the split sensor system needs dual complex amplifying circuits of several stages before comparing two faint sensor outputs, i.e., double in number as compared with a single sensor system. Light pulses are used for the measurment for the purpose of distinction from extraneous light sources, and the number of discrete components such as capacitors needed for the process of a.c. signals is also increased. In addition, the nonlinear amplifier for a wide coverage of input intensity, band-pass filter and synchronous detector are all needed in dual for two sensors.
There is a proposal of reducing the number of circuit components by provision of a multiplexer so that two sensor outputs are processed by one set of circuits on a time-sharing basis. However, the multiplexer creates noises which exacerbate an even worse S/N ratio due to extremely faint reflection light beam from the object as mentioned above, resulting in a degraded performance of range finding, particularly, in measuring a distant object.